In the manufacture of printed circuit boards, sometimes known as printed wiring boards, it has become commonplace to produce printed circuitry on both sides of a planar rigid or flexible insulating substrate. Of increased importance is the manufacture of multilayer printed circuits, the most common of boards sold today in view of increased operational demands for the products in which such boards are implemented. In these boards, the board typically consists of parallel, planar, alternating inner layers of insulating substrate material and conductive metal. The exposed outer sides of the laminated structure are provided with circuit patterns as with double-sided boards, and the metal inner layers typically contain circuit patterns, except in the case of internal power planes which are substantially solid, albeit also containing clearance openings or other openings if desired.
In double-sided and multilayer printed circuit boards, it is necessary to provide conductive interconnections between the various conductive layers or sides of the board. This is commonly achieved by providing metallized, conductive thru holes in the board which communicate with the sides and layers requiring electrical interconnection. For some applications, it is desired that electrical connection be made with almost if not all of the conductive layers. In such a case, thru-holes are also typically provided through the entire thickness of the board. For these, as well as other applications, it is often desired to also provide electrical connection between the circuitry on one face of the board and one or more of the inner circuit layers. In those cases, “blind vias”, passing only part way through the board are provided. In still another case, such multilayered boards often require internal “vias” which are located entirely within the board's structure and covered by external layering, including both dielectric and conductive. Such internal “vias” are typically formed within a sub-part structure of the final board and then combined with other layers during final lamination of the board. For purposes of this application, the terms “conductive thru-hole” is meant to include both thru holes that pass entirely through the board (also referred to in the printed circuit field as plated thru holes or PTHs), “blind vias” which extend from an external surface of the board into a specified conductive layer of the board, as well as an “internal via” which is internal “captured” by the board's outer layers.
To provide the desired circuit pattern on the board, the art has developed a variety of manufacturing sequences, many of which fall into the broad categories of “subtractive” or “additive” techniques. Common to subtractive processes is the need to etch away (or subtract) metal to expose substrate surface in areas where no circuitry is desired. Additive processes, on the other hand, begin with exposed substrate surfaces (or thin commoning metallization layers for additive electroplate) and build up thereon of metallization in desired areas, the desired areas being those not masked by a previously-applied pattern of plating resist material (e.g., called photoresist in the printed circuit board field).
Typically, thru-holes are drilled (including mechanically or more recently using lasers) or punched into or through the board at desired locations. Drilling or punching provides newly-exposed surfaces including via barrel surfaces and via peripheral entry surfaces. The dielectric substrate, comprising a top surface, a bottom surface, and at least one exposed via hole surface, consisting partly or entirely of insulating material, is then metallized, generally by utilization of electroless metal depositing techniques, albeit other deposition processes are also known in the field.
In the manufacture of circuitized printed circuit boards, a dielectric sheet material is employed as the base component for the substrate. This base component typically is an organic material, such as fiberglass-reinforced epoxy resin (also referred to in the field as, simply, “FR4”), polytetrafluoroethylene (e.g., Teflon, a trademark of E.I. DuPont deNemours Company), Driclad (a trademark of Endicott Interconnect Technologies, Inc.), etc. Since the dielectric substrate is nonconductive, in order to plate on the substrate, the substrate is typically “seeded” and plating then occurs. Such processing is known in the field and further description is not believed necessary, except to add that known metals used for plating the dielectric barrel to form the thru holes include copper, nickel and gold.
Examples of methods of making boards, including providing same with such thru holes, are shown and described in the following U.S. Letters Patents:
6,015,520Method For Filling Holes in Printed Wiring Boards6,073,344Laser Segmentation of Plated Through-Hole SidewallsTo Form Multiple Conductors6,188,027Protection of a Plated Through Hole From Chemical Attack6,349,871Process For Reworking Circuit Boards6,493,861Interconnected Series of Plated Through Hole Vias andMethod of Fabrication Therefor6,626,196Arrangement and Method For Degassing Small-High AspectRatio Drilled Holes Prior To Wet Chemical Processing6,628,531Multi-Layer and User-Configurable Micro-PrintedCircuit Board6,630,630Multilayer Printed Wiring Board and Its ManufacturingMethod6,630,743Copper Plated PTH Barrels and Methods For Fabricating6,631,558Blind Via Laser Drilling System6,631,838Method For Fabricating Printed Circuit Board6,638,690Method For Producing Multi-Layer Circuits6,638,858Hole Metal-Filling Method
Additional examples of multilayered printed circuit boards are described and shown in the following published patents applications:
US 2002/0100613 A1Conductive Substructures Of A MultilayeredLaminateUS 2002/0108780 A1Multilayered LaminateUS 2002/0148637 A1High Performance Dense Wire For PrintedCircuit Board
The present invention represents a new and unique method of forming conductive thru holes in a printed circuit board in comparison to those above and other processes known in the art. It is believed that such a method, and the board resulting therefrom, will represent a significant advancement in the art.